Exploration drilling typically involves drilling to subterranean depths of thousands of meters. Accordingly, it is necessary to join and install successive sections of pipe or rod as the drill string is advanced into the well.
Drill rods, depending on their specific configuration, may weigh between ten to fifty kilograms each and measure approximately two to three meters in length. Conventionally, the drill rods are interconnected by male and female threaded connections provided at the respective rod ends. Additionally, it is typically unavoidable to have to exchange the drill bit or other tools at the lowermost end of the drill string at regular intervals during drilling. This exchange process involves retrieving the entire string from the borehole, exchanging the lowermost portion and then reinstalling the entire string after which drilling may continue. In practice, and depending upon rock conditions, it is not uncommon for ten to twenty retrieval operations to be undertaken per drill hole. Accordingly, a very large number of drill rods are required to be handled and in particular taken from a transport or storage carriage to the drilling rig where they are ready for axial alignment and coupling to the drill string. Of course, the reverse operation is also required during string retrieval. Example rod handling systems are described in U.S. Pat. No. 3,043,619; GB 2334270; WO 00/65193; and WO 2011/129760.
A rod handling system may typically comprise a robotic arm having a dedicated gripper for gripping the drill rods. During a forward drilling operation, the robotic arm is arranged to pick-up drill rods at a transport or intermediate carrier and to place the drill rod in the drill rig, whereupon the drill rod is connected to an already installed drill rod to extend the string. During a string retrieval operation, the robotic arm is arranged to pick up disconnected rods from the drill rig and to replace them onto the transport or intermediate carrier.
In order to provide a fully automatic system, that eliminates the need for regular manual intervention, it is desirable for the rod handling system to be able to connect and disconnect the drill rod to/from the installed drill rods. Typically, an end rod of the drill string is engaged by an aft located gripper unit having one or a plurality of rod engagers at least one of which is configured to apply a torque force to the rod via frictional contact with the outside surface of the rod. The remaining rods of the drill string are held by a rod holder at a forward position on the rig. It is advantageous to increase the frictional contact between the rod engagers and the rod so as to ensure the rod is held firmly and a sufficient amount of drive can be translated to impart the rotational movement to the rod. A number of attempts have been made to try and improve the frictional contact between a gripper unit and a rod and examples are disclosed in U.S. Pat. No. 5,221,099; CA 2459628 and US 2005/0188793. However, conventional rod engagers are typically designed for use with a rod of specific and pre-determined diameter and/or are not configured to impart rotational drive to the rod but to grip and hold the rod only. Accordingly, existing attempts either exhibit non-uniform performance when used with rods of different diameter or are incompatible with a gripper unit configured for rod makeup or breakout operations. Accordingly, what is required is rod gripping apparatus that addresses these problems.